A map may be updated to require link addition or deletion and cause a difference in map data before and after a link is added or deleted. In such a case, a map difference data distribution apparatus conventionally generates the difference as map difference data and distributes the map difference data to a vehicle navigation system. For example, patent document 1 discloses the technology that generates map difference data on an area basis and distributes the map difference data to a vehicle navigation system.
Patent Document 1: JP-A-2008-249798 (corresponding to US 2010/0030460)
However, the method disclosed in patent document 1 generates map difference data on an area basis and is likely to update information unneeded for a user. There is a possible problem to increase the traffic (communication time) for map difference data distributed to the vehicle navigation system from the map difference data distribution apparatus. There is another possible problem to unsuccessfully ensure a road network. For example, there may be a road across two areas. The road may be displayed discontinuously if one of the areas is updated and the other is not updated.